1. Field of the Invention
The present invention relates to an organic electroluminescent display (OELD) device, and more particularly, to a top emission type OELD device with a high luminance and method for fabricating the same.
2. Discussion of the Related Art
In general, organic electroluminescent display (OELD) devices emit light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating excitons, and transforming the excitons of an excited state to a ground state. Unlike liquid crystal display (LCD) devices, OELD devices do not require an additional light source and therefore have the advantages of slimness and lightweight.
Since OELD devices have excellent characteristics, such as low power consumption, high luminance, fast response time, lightweight and so on, OELD devices can be applied to various electronic products, such as mobile phones, PDAs, camcorders, plam PCs, and so on. Moreover, due to their simple fabricating process, the fabrication costs of OELD devices are low as compared with LCD devices.
The OELD devices are divided into a passive matrix type and an active matrix type according to the driving method thereof. The passive matrix type OELD devices have a simple structure and a simple fabricating process. However, the passive matrix type OELD devices have disadvantages of high power consumption and low quality images. On the other hand, the active matrix type OELD devices have advantages of high emission efficiency and high quality images.
FIG. 1 is a cross-sectional view illustrating an active matrix type OELD device according to the related art.
Referring to FIG. 1, the OELD device 10 includes first and second substrates 12 and 28 facing each other. The first substrate 12 is transparent and flexible. The first substrate 12 has an array element 14 including a plurality of thin film transistors (TFTs) T and an organic electroluminescent diode E including a first electrode 16, an organic luminescent layer 18 and a second electrode 20. The organic luminescent layer 18 in each pixel region P includes one of red, green and blue color materials.
The second substrate 28 includes a moisture absorbent 22 of a powder type. The moisture absorbent 22 removes moisture inside the OELD device. The moisture absorbent 22 is in a concave portion of the second substrate 28 and is sealed by a taping 25. The first and second substrates 12 and 28 are attached to each other with a seal pattern 26.
In the OELD device, because the first electrode 16 is formed of a transparent material, the light emitted from the organic luminescent layer 18 travels toward the first substrate 12. Accordingly, it is referred to as a bottom emission type OELD device.
FIG. 2 is a circuit diagram of an OELD device according to the related art.
Referring to FIG. 2, gate and data lines 42 and 44 are formed on a substrate 32. The gate and data lines 42 and 44 cross each other and a switching element Ts is formed near the crossing portion of the gate and data lines 42 and 44. The switching element Ts includes a gate electrode 46, a source electrode 56 and a drain electrode 60. The gate electrode 46 is connected to the gate line 42. The source electrode 56 separated from the drain electrode 60 is connected to the data line 44.
A driving element Td is electrically connected to the switching element Ts. The driving element Td of a p-type TFT includes a gate electrode 68, a source electrode 66 and a drain electrode 63. The gate electrode 68 of the driving element Td is connected to the switching element Ts. A storage capacitor Cst is formed between the source and gate electrodes 66 and 68 of the driving element Td. The drain electrode 63 of the driving element Td is connected to the first electrode 16 (of FIG. 1) of the organic electroluminescent diode E. The source electrode 66 of the driving element Td is connected to a power line 55.
When a gate signal from the gate line 42 is supplied to the gate electrode 46 of the switching element Ts, a data signal from the data line 44 is supplied to the gate electrode 68 of the driving element Td through the switching element Ts. Then, the organic electroluminescent diode E is driven by the driving element Td such that the organic electroluminescent diode E emits light. Because the storage capacitor Cst maintains a voltage level of the gate electrode 68 of the driving element Td, even if the switching element Ts is turned off, the organic electroluminescent diode E can continuously emit light for a predetermined period of time.
The switching element Ts and the driving element Td include a semiconductor layer of one of amorphous silicon and polycrystalline silicon. When the semiconductor layer is formed of amorphous silicon, the switching element Ts and the driving element Td can be easily fabricated.
FIG. 3 is a plan view illustrating an array element of an active matrix type OELD device according to the related art and FIG. 4 is a cross-sectional view taken along the line IV-IV of FIG. 3.
Referring to FIGS. 3 and 4, the active matrix type OELD device includes a switching element Ts, a driving element Td and a storage capacitor Cst on a substrate 32. Each pixel of the OELD device may include more than one pair of the switching element Ts and the driving element Td.
A gate line 42 and a data line 44 are formed on the substrate 32 with a gate insulating layer interposed therebetween. A pixel region P is defined by the crossing between the gate and data lines 42 and 44.
The switching element Ts includes a gate electrode 46, an active layer 50 and source and drain electrodes 56 and 60. The gate electrode 46 of the switching element Ts is connected to the gate line 42, and the source electrode 56 of the switching element Ts is connected to the data line 44. The drain electrode 60 of the switching element Ts is connected to a gate electrode 68 of the driving element Td through a gate contact hole 64.
The driving element Td includes the gate electrode 68, an active layer 62 and source and drain electrodes 66 and 63. The source electrode 66 of the driving element Td is connected to a power line 55 through a power line contact hole 58. The drain electrode 63 of the driving element Td is connected to a first electrode 36 through a drain contact hole 65. The storage capacitor Cst includes the silicon pattern 35 as a first storage electrode, the power line 55 as a second storage electrode and a dielectric layer therebetween.
As illustrated in FIG. 4, an organic electroluminescent diode E includes the first electrode 36, an organic luminescent layer 38 and a second electrode 40. The first electrode 36 contacts the drain electrode 63 of the driving element Td through the drain contact hole 65, and the organic luminescent layer 38 is interposed between the first and second electrodes 36 and 40. The first and second electrodes 36 and 40 function as anode and cathode, respectively.
FIG. 5 is a cross-sectional view illustrating an organic electroluminescent diode according to the related art.
Referring to FIG. 5, the organic electroluminescent diode E formed on a substrate 32 includes a first electrode 36, an organic luminescent layer 38 and a second electrode 40. Although not shown, the substrate 32 includes an array element including the driving element Td (of FIG. 4). The first electrode 36 is connected to the driving element Td (of FIG. 4). The first and second electrode 36 and 40 function as anode and cathode, respectively. The organic luminescent layer 38 includes a hole injection layer (HIL) 38a, a hole transporting layer (HTL) 38b, an emitting material layer (EML) 38c, an electron transporting layer (ETL) 38d and an electron injection layer (EIL) 38e. The HTL 38b and the ETL 38d serve to improve emitting efficiency, and the HIL 38a and EIL 38e serve to reduce energy barrier in injecting electrons and holes.
The second electrode 40 functioning as cathode is formed of a low work function material, such as calcium (Ca), aluminum (Al), magnesium (Mg), silver (Ag) and lithium (Li), and the first electrode 36 functioning as anode is formed of a transparent conductive material such as indium-tin-oxide (ITO).
A sputtering process is generally used to form an ITO layer. However, it is difficult to deposit an ITO layer on the organic luminescent layer 38 because of damage on the organic luminescent layer 38 caused by the sputtering process. Accordingly, the OELD device according to the related art is the bottom emission type in which the first electrode 36 of ITO functioning as an anode is formed under the organic luminescent layer 38. However, the bottom emission type has disadvantages of low luminance and low aperture ratio. Moreover, because the first electrode 36 of ITO functioning as an anode is directly connected to the driving element Td, a p-type polycrystalline TFT should be used for the driving element Td, thereby complicating the fabrication process of the OELD device.